1) When the wine has sediment. You gently pour out the wine from the bottle leaving the sediment behind. Such sediment suggests the wine is aged and doesn't need further airing by vinturi

2) When the wine is young and you want to air it - if you're going to decant it then you don't need vinturi

3) For aesthetic reasons: because you don't want a maybe dirty bottle on the table/you prefer the elegance of a decanter/ you don't want to reveal the identity of the wine to guests - if you're going to decant it then you don't need vinturi.

As far as I can see, vinturi is another pointless gizmo. If you need to air and can't decant, just pour the wine into the glass from a bit higher and wait a bit.

If you have a couple of hours to follow a bottle as it airs/progresses then just leave it alone - no decanting or other aerating method.

I have seen the vinturi type devices work to a limited degree. IMO, they only really come in handy at dinners where things are flying around pretty fast, so there's no time to let the wine breathe in the glass.

I rarely decant and I don't own a vinturi. I will decant vintage port for the sediment reason that Peter gives you. Almost everyting else is pop and pour. Rarely, I will taste a wine that was PnP and decide to put it into a decanter for a little extra air but for the most part, I prefer to experience the wine change over the course of a few hours.

I should note that I rarely drink wines that are considered too young these days. I found that I prefer most wines with some age on them which may factor into how few wines see a decanter in my house.

I have tested the Vinturi and it does work immediatley, both on the nose and on the palate. However, I do not own one. I either decant for all the reasons Peter listed, or I pour into wine glasses and let them air, as David indicated, taking a sip once in awhile to witness the wine evolve (I usually do this while preparing dinner). To take the guess work out of the decision, I decant practically every red wine I open with one guideline: older wines may need less time to fully open up and, if left to long, may degrade before you know it.Unlike Peter, I always place the wine bottle on the table next to the decanter no matter how unattractive it may look.

I have a Venturi. I agree with Jon. I also run across some bottles that are reductive and have found that putting a penny (pre-1983) in the screen eliminates these odors and will greatly improve the wine. I keep a penny stored with mine.

I would decant with lots of sediment, if I had the time or patience, but I don't. Never used or owned a vinturi or any other aerator. I just do the best I can with the bottle, and suck down lots of sediment on the last glass or 2. It's the price I pay for being lazy.

I am looking at years since vintage. Adjusted for what the wine is, where it came from (region and producer), and vintage characteristics. Not a perfect science by any means. For example, I learned long ago that I prefer even my cali Pinot with 7 or more years on them so at that age, they rarely need decanting but I do get the occasional wine that should have been consumed earlier. Less than one might guess however for my tastes. I will adjust that for say Arcadian which I know ages differently and hold those longer as my 2005s continue to sleep.